1. Field of the Invention
The present invention relates to a method for improving surface luster of a metal sheet at the time of cold-rolling and a method for cold-rolling metallic materials so as to improve glossinesses of the both surfaces of metallic materials.
2. Description of the Prior Art
In recent years, demands of users for qualities of rolled sheets of various metals as represented by thin stainless steel sheets (hereinafter described simply metal sheets) have been becoming more and more severe. Above all, with respect to thin stainless steel sheets, those having an especially high glossiness have been demanded.
Luster of a metal sheet surface is influenced mainly by an amount of lubricating oil introduced between a roll and a metallic material during cold-rolling. If amount of lubricating oil is too much, the surface of the metallic material is freely deformed by its static pressure, resulting in occurrence of fine recessed flaws called oil pits, and a glossiness is lowered. Also in the case where lubricating oil having a low viscosity is used or a small amount of lubricating oil is used and thereby metallic contact portions between a metal sheet and a roll are increased, a problem such that a seizure flaw is produced, becomes liable to occur.
Heretofore, as one of cold rolling methods for a metal sheet, a rolling method making use of a cross-roll mill has been known. This rolling method was such method that a pair of work rolls for use in rolling are disposed so as to cross with each other as inclined in the opposite directions to each other with respect to a direction at right angles to a feed direction of a metal sheet forming a material to be rolled, and rolling is effected by pinching and pressing the metal sheet with these work rolls.
And at the time of rolling a metal sheet through such a rolling method, not only a sheet configuration but also quality of luster of the metal sheet surface were taken seriously as a part of quality of the product, but it was difficult to satisfy the both requirements.
In addition, at present, for the purpose of obtaining a metal sheet having a high glossiness, a cold rolling method making use of a mill called "Sendzimir mill" is generally practiced. In this Sendzimir mill, since a diameter of work rolls is small and a rolling speed is slow, excessive lubricating oil would not be introduced into a roll caliber tool, and a metal sheet having a high glossiness can be manufactured. However, cold-rolling by making use of a Sendzimir mill involves the problem that it is inefficient because a rolling pass is repeated by a lever system and a rolling speed is slow due to a small diameter of rolls.
Hence, an attempt at producing metal sheets having a high glossiness more efficiently by making use of a tandem mill capable of carrying out high-speed rolling, has been done. However, if high-speed rolling is effected with a tandem mill having a large roll diameter, there is a problem that an introduced amount of lubricating oil increases and a glossiness is lowered. In order to resolve this problem, in the official gazette of Laid-Open Japanese Patent Specification No. 61-49701 (1986) is disclosed a cold-rolling method, in which after cold-rolling has been carried out by means of a tandem mill provided with work rolls having a large diameter of 150 mm .phi. or more, finish rolling is effected by making use of a Sendzimir mill employing small-diameter rolls of 100 mm .phi. or less as work rolls, and thereby a thin stainless steel sheet having few surface defects can be obtained. However, since this method necessitates two kinds of installations of a tandem mill and a Sendzimir mill and moreover eventually a Sendzimir mill is used, there still remains a problem that a rolling speed is limited and a productivity is not improved.
On the other hand, in the case of cold-rolling metallic materials, there exists a problem that luster of a metal sheet is different between its upper surface and lower surface because of the fact that an amount of adhesion of lubricating oil does not become equal between the upper and lower surfaces of the metallic material. In general, an upper surface which is rolled under a condition rich in lubricating oil, becomes to have a smaller glossiness. Therefore, in the official gazette of Laid-Open Japanese Patent Specification No. 55-165217 (1980) is disclosed a method for rolling by changing a pass-angle of a metallic material. This method is a rolling method improved so as to reduce an amount of introduced lubricating oil by enlarging a biting angle of an upper surface, but it involved a problem that a space for newly equipping an additional device was necessitated for a mill.
Generally it is known that for the purpose of giving excellent luster to a metal sheet, work rolls to be used for rolling had better have a smaller surface roughness, and a technique of improving luster by making use of work rolls having a large surface roughness is not known.
In the above-described method in the prior art of selecting lubricating oil having a low viscosity or reducing an amount of lubricating oil introduced between a metal sheet and a work roll by making use of work rolls having a small diameter, since a lubricating condition between the metal sheet and the work rolls is deteriorated, the problem that the metal sheet would be overheated due to friction between the metal sheet and the work rolls and seizure would be generated, is liable to occur. Therefore, in the prior art, in the case where it is intended to improve surface luster of a metal sheet, it was necessary to work with a rolling speed lowered.
Therefore, in the industry of cold-rolling a product whose surface luster is deemed to be an important merchandise value such as stainless steel sheets, aluminium sheets, etc., establishment of a rolling method which can improve surface luster while enhancing a productivity was a long-standing problem.